Among cellulose films, a cellulose acetate film has higher optical isotropy (a lower retardation value) than other polymer films. Therefore, it is common to use a cellulose acetate film in applications that require optical isotropy, e.g. for a polarizing plate.
On the other hand, it is demanded that optical compensation sheets (retardation films (phase-contrast films, or phase difference films)) for use, for example, in a liquid crystal display device, have optical anisotropy (a higher retardation value). Therefore, a synthetic polymer film, e.g. a polycarbonate film or polysulfone film, which has a high retardation value, is generally used as the optical compensation sheet.
Meanwhile, in recent years, there is proposed a cellulose acetate film which has such a high retardation value that it can also be used in applications that require optical anisotropy (see, for example, JP-A-2002-296421 (“JP-A” means unexamined published Japanese patent application)). In JP-A-2002-296421, a compound described in paragraphs 0055 to 0064 is added before stretching treatment of a cellulose acetate film, for providing it with high retardation value. However, such a compound may precipitate (bleed out) on the surface of the film, depending on its compatibility with cellulose acetate, causing a concern about durability of the film.